Autofocus method for microscope and microscope comprising autofocus device

ABSTRACT

A microscope including an objective having a focal plane in a sample space, and an autofocus device comprising a light modulator for generating a luminous modulation object that is intensity-modulated periodically along one direction, an autofocus illumination optical unit that images the modulation object such that its image arises in the sample space, an autofocus camera, an autofocus imaging optical unit that images the image of the modulation object in the sample space onto the autofocus camera, a control device, which receives signals of the autofocus camera and determines an intensity distribution of the image of the modulation object and generates a focus control signal therefrom. The control device determines an intensity distribution of the image of a luminous comparison object imaged by the optical unit to correct the intensity distribution of the image of the modulation object with regard to reflectivity variations in the sample space.

PRIORITY CLAIM

The present application is a National Phase entry of PCT Application No.PCT/EP2012/060605, filed Jun. 5, 2012, which claims priority from GermanApplication Number 102011077236.7, filed Jun. 8, 2011, the disclosuresof which are hereby incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The invention relates to a microscope with an objective, which has afocal plane lying in a sample space, and an autofocus device having alight modulator which is designed for generating a luminous modulationobject that is intensity-modulated periodically along one direction, anautofocus illumination optical unit which images the modulation objectsuch that an image of the modulation object forms in the sample space,an autofocus camera, an autofocus imaging optical unit which images theimage of the modulation object formed in the sample space onto theautofocus camera, a control device which receives signals of theautofocus camera and is designed to determine an intensity distributionof the image of the modulation object and to generate a focus controlsignal therefrom.

The invention further relates to an autofocus method for a microscopewith an objective which images a sample, wherein a luminous modulationobject that is intensity-modulated periodically along one direction isimaged onto the sample such that an image of the modulation object formsthere, an intensity distribution of the image of the modulation objectis determined and a focus control signal is generated therefrom.

BACKGROUND OF THE INVENTION

A microscope according to the preamble and an autofocus method accordingto the preamble are known from WO 2007/144197 A1. This describes anautofocus device which images a modulation object that isintensity-modulated periodically along one direction into the sample andgenerates an autofocus signal from the image of the modulation object.Similar microscopes are found in DE 102006027836 A1 and DE 102007055530A1.

This approach achieves very good autofocus properties, but occasionallyhas problems when the reflective properties of the sample vary locally,because then the image of the modulation object can be distorted by thereflection variations of the sample. In the worst case, the image of themodulation object can be altered by the object image so radically thatan autofocus signal can no longer be generated.

DE 19537376 A1 uses two patterns from periodic grids, phase-shifted by180°, in different focal planes. The patterns serve to determinedirection for the autofocusing.

DE 102008005355 A1 describes the effect a reflectivity jump has on theAF contrast signal. To thereby avoid negative influences, a defocusingof the projected grating structures is proposed.

In U.S. Pat. No. 4,725,722, differently defocused grating images aresubtracted from each other.

SUMMARY OF THE INVENTION

The object of the invention is to develop a microscope or an autofocusmethod of the type named at the beginning such that, even in the case ofsamples which show a reflection variation, a reliable autofocus functionis obtained.

This object is achieved with a microscope of the above-mentioned type inthat the light modulator additionally generates a luminous comparisonobject which extends along the direction of the modulation object, theautofocus illumination optical unit also images the comparison objectsuch that an image of the comparison object forms in the sample space,the autofocus imaging optical unit also images the image of thecomparison object formed in the sample space onto the autofocus camera,and the control device is designed to determine from the signals of theautofocus camera an intensity distribution of the image of thecomparison object along the direction and thereby to correct theintensity distribution of the image of the modulation object with regardto reflectivity variations in the sample space.

The object is further achieved with an autofocus method of theabove-mentioned type in which a luminous comparison object which extendsalong the direction of the modulation object is additionally imaged ontothe sample such that an image of the comparison object forms there, anintensity distribution of the image of the comparison object along thedirection is determined and the intensity distribution of the image ofthe modulation object is thereby corrected with regard to reflectivityvariations of the sample.

According to the invention, a comparison object is provided which isalso imaged to the sample. The intensity distribution of the image isdetermined along the direction of the reference object. The intensitydistribution of the image of the comparison object can be used to easilycorrect the intensity distribution of the image of the modulation objectwith regard to reflectivity variations in the sample space.

The comparison object thus allows to determine reflection variations inthe sample space, which could distort the imaging of the modulationobject. For this, the comparison object is expediently arranged alongto, in particular aside, the modulation object and, unlike themodulation object, is not intensity-modulated periodically, so that theintensity distribution of the image of the comparison object along thedirection of the image of the modulation object reflects any reflectionvariations in the sample space.

This can be achieved particularly simply in that the comparison objecthas the form of a line or a rectangular field. A required lack ofperiodic intensity modulation can also be achieved by forming thecomparison object with a fineness of structure which cannot be resolvedby the microscope.

Ideally, the comparison object extends along the direction of themodulation object over precisely the same length that the modulationobject has. The same is then also true for the corresponding images ofthese objects. The microscope or the autofocus method then allows acorrection over the whole range of extension of the modulation object.

In principle, it is advantageous if the comparison object is designedsuch that the total intensity of the luminous comparison object is equalto that of the modulation object. This can be achieved, e.g. by theluminous surface area of the comparison object corresponding to that ofthe modulation object. If the luminous line or the luminous rectangularfield is used as a comparison object, the width of the comparison objectis preferably by a factor smaller than that of the modulation objectwherein the factor equals the duty cycle of the modulation object. Afurther option to make the intensity of the image of the comparisonobject conforming as far as possible to that of the image of themodulation object is to form the comparison object as a grating slit,the grating constant of which lies below the resolution limit of theautofocus imaging optical unit. In both cases, the comparison objectappears reduced in its intensity. The reduction conforms exactly to theintensity of the modulation object if duty cycle and grating slit widthof modulation object and comparison object are substantially the same orthe width of the comparison object is smaller by a factor equalling theduty cycle of the modulation ratio.

A further possibility for forming the comparison object is to provideadditionally to the modulation object a structure that complements themodulation structure, e.g., a replica of the modulation object with aphase position shifted by 180 degrees. The comparison object is then thecombination of modulation object and the complementary structure.

The imaging of comparison object and modulation object can preferablytake place simultaneously. A simplification in the autofocus camera isachieved if the images are imaged sequentially and determinedsequentially.

It is understood that the features mentioned above as well as those yetto be explained below can be used, not only in the stated combinations,but also in other combinations or alone, without departing from thescope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained yet more fully in the following, by way ofexample, with reference to the drawings. There are shown in:

FIG. 1 a microscope with an autofocus device for reflectionmeasurements,

FIG. 2 a microscope with an autofocus device for fluorescencemeasurements,

FIG. 3 a microscope similar to that of FIG. 1, adapted for samples thatscatter,

FIG. 4 a microscope similar to that of FIG. 1 with an increased capturerange,

FIG. 5 an inverted microscope with an autofocus device with reflectionof the illumination between objective and filter cube,

FIG. 6 an inverted microscope with an autofocus device with reflectionin the microscope illumination beam path,

FIG. 7 a microscope similar to that of FIG. 6, with an autofocus devicein which intermediate imaging optics is arranged in front of a camera,

FIG. 8 a top view of an exemplary light modulator, and

FIGS. 9 to 11 contrast signals to illustrate the functioning of theautofocus device.

DETAILED DESCRIPTION

FIG. 1 shows a microscope 1, with which a sample 2 which is located on asample holder 3 is imaged by means of an objective 4 onto a detector 5.The basic structure of the microscope corresponds to that of themicroscope known from WO 2007/144197 A1. This applies to the designs ofFIGS. 1-7. The disclosure of this published document is therefore fullyincorporated by reference herein. The difference compared with themicroscopes of WO 2007/144197 A1 lies in the structure of the lightmodulator 12, which will be explained yet more fully in the followingwith reference to FIG. 8.

The representation in FIG. 1 is greatly simplified. The microscope 1 canbe a widefield microscope, i.e., the detector 5 is, e.g., a camera or aneyepiece. However, any other microscope design also comes intoconsideration for the microscope 1, for example a laser scanningmicroscope. Then, the microscope beam path is provided with anotherscanning device which lies on the optical axis OA1. The focal positionof the objective 4 is adjustable, as indicated by the double arrow.Alternatively, the sample holder 3 can also be adjustable.

A beam splitter 6 which can optionally be dichroic or a plate-typecolour splitter couples in microscope illumination radiation from amicroscope illumination source 8 which illuminates the sample 2 throughthe objective 4 and an illumination tube lens system 7. By means of abeam splitter 11 an autofocus device is coupled into this illuminationbeam path. The autofocus device has a light modulator 12 which isilluminated by a light source 13, for example, an LED, when operating intransmission or by a light source 14 (e.g. an LED), when operating inreflection. The illuminated light modulator 12 generates a modulationobject. This is projected, i.e., imaged, to the sample 2 via the beamsplitter 11, the illumination tube lens system 7, the beam splitter 6 aswell as the objective 4. This realizes an autofocus illumination. Theimage of the modulation object generated in the sample 2 is recorded inthe opposite direction by means of a camera 16 in front of which afurther beam splitter 15 is located on the optical axis OA2 of theautofocus device. The thus obtained optical axis OA3 is optionallyfurther guided onto a camera 18 via a beam splitter 17.

In an embodiment of the invention, the beam splitter 11 is dichroic, andthe light source 13 or 14 emits at a wavelength which is not emitted toa significant extent by the microscope illumination source 8 or is notnecessary for the sample imaging. The autofocus device thus operates ina spectral range which is not otherwise used for the imaging of thesample 2.

In the figures, generally only the image planes of the cameras areshown. The cameras can generally be CCD or CMOS cameras.

In the design of FIG. 1, the light modulator 12 and thus the modulationobject lie perpendicular to the optical axis OA2. The image plane of thecamera 16, on the other hand, is at an angle to the optical axis OA3. Ifthe light modulator 12 provides for example for a spatial modulation,e.g. a striped pattern, the maximum contrast is located in a line of thecamera 16 lying perpendicular to the plane of drawing. The position ofthe line on the camera 16 is a measure of the position of the focalplane along the optical axis, i.e., in z-direction.

With the help of the illumination tube lens system 7 and the objective4, the modulation object which is generated by the light modulator 12either in transmission operation (light source 13) or in reflectionoperation (light source 14) is imaged into the sample. The camera 16positioned at an angle obtains a depth resolution. By means of thecamera 18, a lateral shift of the sample illuminated in a structuredmanner can additionally be detected.

In the case of a fluorescence microscope, an excitation filter 10 and anemission filter 9 (for the fluorescence radiation) are used. Withoutfurther limitation with regard to the autofocus device, the microscope 1can also be designed as a scanning microscope, in particular as a laserscanning microscope, Nipkow disk microscope or SPIM microscope.

FIG. 2 shows an alternative design of the microscope of FIG. 1 withregard to the autofocus device. In the microscope of FIG. 2, componentswhich have already been explained with reference to FIG. 1 are providedwith the same reference numbers; repeated description thereof istherefore dispensed with. The camera 16 here detects the fluorescencecontrast of the sample, which can be further enhanced by the structuredautofocus illumination. The imaging of the modulation object is donesimilarly to the design according to FIG. 1. However, the modulationimage imaged to the sample 2 is not defected from the illumination beampath, but via a separate beam splitter from the microscope beam path.This splitter is adapted to the wavelength of the autofocus illuminationand, e.g., is dichroic to the same extent as the beam splitter 11. Inorder to be able to record the modulation object imaged to the sample orreflexes or backscatterings generated thereby by means of the camera 16,the camera is preceded by a corresponding relay optical unit 20, theoptical characteristics of which unit ensure that the image plane of thecamera 16 intersects a plane conjugated to the modulation object,ideally close to or on the optical axis (as also FIG. 1).

The signals supplied by the auto focus device, in particular from thecamera 16, are utilized for the control of a focus adjustment along thez-axis. This is illustrated schematically in FIG. 1 by a double arrow.The control device used is not shown in FIGS. 1 and 2.

If the microscope 1 of FIG. 2 is designed as a laser scanningmicroscope, the combination of emission filter 9 and excitation filter10 is again provided, as well as optionally a further spectral filter 21which ensures that the spectral range of the luminous modulation objectfalls onto the camera 16 and spectral ranges that are of no furtherinterest are blocked, in particular the spectral ranges of the sampleimaging.

FIG. 3 shows an embodiment of the microscope 1 for strongly scatteringand scarcely reflective samples. Elements which have already beenexplained with reference to FIG. 1 or 2 are not described further. Theyare provided with the same reference numbers in the figure.

For strongly light-scattering surfaces, such as, e.g., sections oftissue, the autofocus device is modified according to FIG. 3. Thisdevice predominantly analyses the light scattered by the sample.

In this arrangement, the two-dimensional light modulator 12 encloses anangle of between 0 degrees and 90 degrees, preferably between 20 degreesand 70 degrees, with the optical axis OA2. The light modulator 12 againcorresponds to the already described design, i.e., it can be atransmission LCD, a reflection LCD, a DMD or an amplitude grating withshifting device. The structure preferably again consists of light darkstripes. The illumination of the light modulator 12 preferably comesfrom one or more high-power LEDs. This is also possible in FIGS. 1 and2. In a transmission type modulator the light source 13 is provided; ina reflection type modulator the light source 14 is provided. Usualoptical devices for light homogenizations and optical units forintermediate imagings can be used to generate the modulation object withthe light modulator 12 and are not represented in FIG. 3 (as also inFIGS. 1 and 2) for reasons of clarity.

With the help of the illumination tube lens system 7 and the objective4, the modulation object is imaged to the sample 2. As the lightmodulator 12 and thus the modulation object are not perpendicular to theoptical axis, the modulation object (e.g., alternating light darkstripes) is imaged at an angle to the optical axis and thus into thedepth of the sample.

The image plane of the camera 18 lies in a plane conjugated to the lightmodulator 12 and thus to the modulation object. It is thus tilted at thesame angle to the optical axis (here the optical axis OA3) as the lightmodulator 12. Light scattered by the sample is hereby imaged, modulated,onto the camera 18. The contrast of the modulation object appears oncamera 18 only where it is scattered by the sample. The embodiment inFIG. 2 is therefore particularly suitable for thin or non-transparent,scattering samples, for example, in technical microscopy. At the sametime, the capture range is enlarged by the tilted modulation object.

For the autofocus function the autofocus device additionally has thecamera 16, the image plane of which is at an angle to the lightmodulator 12 and thus to the modulation object, as it lies perpendicularto the optical axis OA3. It is mirrored in a 50% splitter 17 locatedwithin the autofocus beam path.

FIG. 4 shows a further modification of the microscope with regard to theautofocus device. The arrangement of FIG. 4 differs from that of FIG. 3in that the autofocus beam path is coupled in as close as possible tothe objective pupil.

This alternative is possible in principle for all designs of theautofocus device and provides a separate dichroic beam splitter 11′ inthe microscope beam path which couples in the radiation of the lightmodulator 12 and guides the representation of the modulation objectimaged to the sample onto the camera 16 or 18. In the representation ofFIG. 4, the beam splitter 11 is then dispensed with in this design.Instead, a reflecting mirror 11″ is optionally provided. The coupling inalso takes place via the beam splitter 11 by means of an optionaloptical unit 20 which generates a possibly necessary intermediateimaging and guarantees that the modulation object, i.e., the illuminatedlight modulator 12, lies in a plane conjugated to the focal plane of theobjective 4, therefore the modulation object is imaged through theobjective 4 into the sample 2. This concept is described later withreference to FIG. 7 in another variant.

The design drawn in as an alternative by way of example in FIG. 4 hasthe advantage that the autofocus device can operate very reliablysimultaneous with all current microscopy processes. In particular, forthe modulation object, i.e., the illumination of the light modulator 12,it is possible to use radiation the wavelength of which lies above thewavelengths used for the microscopy processes. In fluorescence ortransmitted light measurements, a wavelength range above 700 nm,preferably above 800 nm, can be used for the autofocus device. Anadditional spectral filter in front of the camera or cameras caneffectively suppress potentially interfering light of the microscopeillumination.

The designs of FIGS. 1 to 4 show the arrangement of the autofocus devicein an upright microscope 1 with an object slide as a sample holder. Acover glass, as well as an immersion fluid (e.g., oil, water, glycerol),can be located between object slide and objective. However, this is notessential. There are thus two cases of application to be differentiated:The highest grating contrast of the air/gas boundary surface on the topside of the cover glass or of the object slide is obtained withoutimmersion fluid. The highest grating contrast of the boundary surfacebetween the underside of the cover glass and the embedding medium isobtained with immersion fluid. To suppress scattered light or undesiredreflexes, diaphragms, e.g., semicircular diaphragms, can be introducedinto the autofocus beam path.

The beam splitter 11 or 11′ can be a plane-parallel glass sheet whichhas an anti-reflexion coating on one side, to preventing interferingsecondary images. The side of the glass sheet serving to reflect theautofocus signal can also have a dichroic coating which increases thereflectivity for the long-wave autofocus light (if the variant withlong-wave autofocus illumination is used) and predominantly transmitsthe shorter-wave useful light of the microscopy (e.g., fluorescenceradiation). Naturally, other spectral partitions are also possible.

FIG. 5 shows an inverted microscope 1 with an autofocus device accordingto the invention. Such inverted microscopes are predominantly used toread titer plates which have cuvettes 22 with liquids which cuvettesopen to the top. The glass/liquid boundary surface of the titer platebase (or close to it), where cells usually adhere, is usually to befocused to. As these cells impair the contrast of the projected grating,i.e., of the modulation object, at least ten lines of thetwo-dimensional camera 16 are preferably evaluated in this design. Thecontrast signal can thereby be averaged over a larger number of lines,which substantially improves the measurement accuracy. Again, air or animmersion fluid can be provided between objective 4 and titer platebase, and what has already been said above applies analogously. The onlydifference is that, instead of the embedding medium, there is a spaciousliquid column above the sample in the cuvette 22.

The autofocus device in the design of FIG. 5 is again coupled into themicroscope beam path via its own dichroic beam splitter 11′, and notinto the illumination beam path. Otherwise, what has already been saidapplies analogously. In addition, the optional spectral filter 21 isalso shown in FIG. 5. The illumination optical unit 23 is alsorepresented by way of example for generating the modulation object fromthe light modulator 12. For a homogeneous illumination of the lightmodulator 12, the illumination optical unit 23 preferably also containsa diffusion disc. An illumination optical unit 23 can be used in allembodiments described.

FIG. 5 further shows that the signals of the camera 16 are fed to acontrol device 26 which carries out corresponding calculations and,among other things, controls the already mentioned z-drive 27 foradjusting the focal position. Naturally, the control device 26 is inmost applications also connected to the light modulator 12, if this iscontrollable. The same applies to the light source 13 or 14. The controldevice 26 and the elements connected can also be present in the designsaccording to FIGS. 1 to 4.

FIG. 6 shows a modification of the design of FIG. 5. Here, the autofocusdevice is mirrored into the illumination beam path via the beam splitter11 analogously to the design of FIG. 1. The separate optical unit 20 canbe dispensed with, as no autofocus illumination tube lens system isnecessary then; its function is fulfilled by the illumination tube lenssystem 7. In addition, in FIG. 6 optional filters 24 and 25 in theillumination beam path are also shown. The coupling in via the beamsplitter 11 also follows in the imaging direction of the modulationobject, a demagnification optical unit 28 which influences the capturerange of the autofocus device and thus makes its ideal design possible.The optical units 7, 20 and 28 can also be designed as (motorized)varifocal optical units, in order to compensate for different objectivemagnifications.

FIG. 7 shows a further design of a microscope with an autofocus device.Components which correspond, in terms of their function and/orstructure, to elements from already described microscopes are again notexplained again here, in order to avoid unnecessary repetitions. In theautofocus device of FIG. 7, the camera 16 follows an imaging opticalunit 29, with the result that the image of the modulation object in thesample is not imaged directly, but an intermediate image 30 is imagedonto the camera 16. Naturally, this principle, which is shown in FIG. 7using the example of an inverted microscope, can be used generally. Thelight modulator designed here as a transmission grating and theintermediate image 30 lie in planes which are conjugated to the samplefocal plane as well as to the plane which the camera 16 intersects.

The imaging of the image of the modulation object lying in the sampleusing the intermediate image 30 has the considerable advantage that thefurther beam splitter 15, which can e.g., be designed as a 50:50 beamsplitter, lies at a great distance from the camera 16.

Any unavoidable scattered light from the further beam splitter 15 thusreaches the camera 16 only via multiple reflections, i.e., greatlyweakened. In addition, the intermediate image coupling avoids allreflexes from optical units which are located to the right of thefurther beam splitter 15, to the camera 16.

In an exemplary design of the autofocus device of the microscope of FIG.7, an IR-LED the centroid wavelength of which lies above 800 nm,preferably at 830 nm, is used as light source 13. The light source 13 isconnected to the control device 26, such that, necessary, the lightsource will be switched on or off, time-modulated or regulated regardingits brightness.

The light modulator 12 is designed as a slit transmission grating. Inorder to illuminate the slits as homogeneously as possible and thus toutilize the radiation of the illumination source 13 as efficiently aspossible, a collimating optical unit with an anamorphic optical unit 23which brings about a linear illumination is provided. The lightmodulator 12 is preceded (alternatively also followed) by the filter 21which is designed as an IR bandpass filter and is adapted to thecentroid wavelength of the IR-LED. The bandpass width lies between 10and 50 nm, whereby undesired spectral portions of the light source 13are suppressed.

At the further beam splitter 15, a light trap 34 is provided whichabsorbs radiation from the light modulator 12 which would betransmitted. The light trap 34 is preferably designed as a sheet of astrongly absorbent material positioned at an angle to the optical axis,e.g., as a polished NG1 sheet. NG1 is a strongly absorbent, black glass.

The dichroic beam splitter 11′ reflects only radiation of the wavelengthwhich propagates after the bandpass filter 21 in the autofocus beampath. Other spectral portions are transmitted. To achieve this, the beamsplitter 11′ has an interference layer which mainly transmits radiationbelow 800 nm at an angle of incidence of 45 degrees and mainly reflectsradiation with the centroid wavelength of the IR-LED. The beam splitter11′ can also have a wedge angle in order to avoid interferences whichwould be disruptive in a laser scanning microscope, which can be oneembodiment of the microscope of FIG. 7. In order to also be able to workwith non-linear fluorescence excitation which also uses IR radiation inthe microscope for sample imaging, the beam splitter 11′ can also have abandpass-like spectral behavior, with the result that wavelengths abovethe autofocus spectral range (e.g., above 840 nm) are also largelytransmitted at a 45-degree angle of incidence. Naturally, these beamsplitter features can also be used in other embodiments.

Furthermore, it is preferably possible to configure the beam splitter sothat it can be changed, e.g., by means of a change wheel or anotherchange mechanism.

The imaging of the image of the modulation object formed in the sampleor the intermediate image 30 thereof is preceded by bandstop filters 33which ensure that only radiation of the corresponding autofocus spectralrange reaches the camera 16. Other radiation which can originate fromthe imaging of the sample, e.g., excitation or fluorescence radiation inthe case of a laser scanning microscope, is thereby suppressed onceagain. The imaging optical unit 29 images the image of the modulationobject formed in the sample or the intermediate image 30 reflected at aboundary surface, e.g., the glass/liquid boundary surface, onto thecamera 16. The imaging optical unit 29 can be constructed, e.g., from atube lens 32 and a standard objective 31 with small numerical aperture(e.g. NA=0.2).

FIG. 8 shows a top view of a design of the light modulator 12 forgenerating the modulation object, which is here designed as a gratingslit G1. The imaging of the grating slit G1 generates striped contoursin the sample. Alongside the grating slit G1, there is additionally afurther grating slit R1 which is designed as a comparison object withwhich the lateral reflectivity distribution of the sample is measureddirectly next to the point at which the grating slit G1 is imaged to thesample. Using the measured reflectivity distribution, the signal, i.e.,the intensity distribution of the imaging of the grating GI, isnormalized in order to eliminate the influence of reflectivityfluctuations of the sample 2 computationally.

The grating slit R1 has a grating frequency which is so high that it canno longer be resolved by the optical unit. The image of the grating slitR1 in the sample 2 is thus not structured. As the duty cycle of thegrating slit RI corresponds to that of the grating slit G1, the amountof light that passes into the sample through the grating slit R1 duringthe imaging is equal to the amount of light that passes onto the sampleduring the imaging of the grating slit G1. The intensity pattern alongthe image of the grating slit G1 can thereby be corrected particularlyeasily by means of the intensity pattern along the image of the gratingslit R1.

The result is shown using FIG. 9. This represents the intensity patternof the reflected radiation of the image of the grating slit G1 as curveS1. It can be seen that the intensity of the reflected radiation fallsfrom a plateau I1 to a value 12 and then rises again. This pattern iscaused by a lower reflectivity of the sample 2 between the points x1 andx2. The image of the grating slit R1 serving as a comparison objectreproduces this pattern exactly (not shown), but without the modulationin the area of point x1. If the signal of the curve S1 is normalizedwith the intensity pattern of the image of the grating slit R1, thesignal according to curve S3 represented in FIG. 11 is obtained. Themodulation in the area of point x1, which is caused by the modulationobject in the form of the grating slit G2, is now clearly recognizableand can be used to generate an autofocus signal.

The top view of the light modulator 12 which is represented in FIG. 8shows yet another grating slit G2 which corresponds to the grating slitG1 apart from a phase shift of 180 degrees. In this embodiment, thenormalization can also take place by a reference to the combinedintensity of the images of the grating slits G1 and G2. This is shown byFIG. 10, which, on a curve S2, shows the signal which is obtained in theevaluation of the intensity pattern along the grating slit G2. Addingthese two signals gives a variable which, when utilized as normalizationfor the signal that was obtained from the image of the grating slit G1or the image of the grating slit G2, also results in the signal patternaccording to curve S3 of FIG. 11. It is essential for this that thegrating slit that is used as modulation object (for example the gratingslit G1) is combined with a structure that complements it (for example,grating slit G2), with the result that the intensity distribution of theimage of the modulation object combined with the image of the structurethat complements it corresponds to a continuous slit. The structure ofthe modulation object repeated at a phase shift of 180 degrees forgenerating a comparison object is to be understood only as exemplary. Inprinciple, it suffices if the structure which, together with themodulation object, forms the comparison object is inverse to themodulation object.

The light modulator 12 of FIG. 8 thus offers the following options foreliminating reflectivity variations of the sample 2 by means ofnormalization:

1. G1/(G1+G2)

2. G2/(G1+G2)

3. G1/R1

4. G2/R1

5. G1/R2

6. G2/R2.

In this list, the reference numbers of the corresponding grating slitsare used as symbols for the intensity distribution of the images ofthese grating slits in the sample 2 measured by means of the autofocuscamera.

Naturally, the intensity distributions can also be provided with asuitable prefactor; for example, in the cases in which the modulationobject is combined with a structure that complements it a correspondingprefactor which takes into account the different light intensity betweenmodulation object and comparison structure can be used in thenormalization. The grating slits shown in FIG. 8 are to be seen asmerely exemplary. In principle, a modulation object with a furtherstructure which, either alone or in combination with the modulationobject, represents the comparison structure suffices. However, the useof additional modulation structures or grating slits, as in FIG. 8,increases the precision and robustness of the method.

The light modulator 12 also need not, as shown in FIG. 8, be designed asa static structure. It can be realized by means of a correspondinglycontrollable element which provides the modulation structure and theadditional structures for the comparison structure. This can also takeplace sequentially in principle. This has the advantage that comparisonstructure and modulation structure can be imaged into the sample 2exactly at the same point, with the result that the inherently smalloffset between comparison structure and modulation structure whichexists for example between the grating slits G1 and R1 of FIG. 8 is alsoeliminated. Thus, it is absolutely impossible for the reflectivity ofthe sample to vary locally between the images of the modulation objectand the comparison object.

The areas on the autofocus camera allocated to the modulation andcomparison objects respectively can preferably consist of severalcolumns and rows, with the result that an average can be taken overseveral columns or rows to minimize noise.

The capture range of the autofocus system is predetermined by the rangeof the z-shift of the objective 4 (or the boundary surface) in which acontrast signal that can still be evaluated is present on the autofocuscamera. Two conditions have to be met for this:

-   -   1. The contrast signal has to be located inside the image field.    -   2. The contrast signal has to have a sufficient signal level        which lies significantly above the noise level.

The holding area in which the focus can be held by readjustment islimited by the maximum allowable range of the z-shift of the focal planeinto the sample. For the case that the contrast focus is located in thecenter of the image field, if the focal plane of the microscopeobjective 4 lies precisely on the boundary surface, the holding area isprecisely half as large as the capture range.

In a digital signal resolution of 8 bit (256 steps), a contrast level of30 can only just be meaningfully evaluated. For this reason, the imagefield size on the autofocus camera positioned at an angle must be chosensuch that, at the edge of the capture range, the contrast level onlyjust suffices. The size of the camera is correspondingly adapted.

A development is therefore optionally provided which is drawn in by wayof example as a dotted line in FIG. 7. A lengthwise adjustment mechanism40 is provided there which varies the distance between autofocusobjective 31 and autofocus camera 16. For this, the objective 31 canoptionally be moved relative to the fixed camera 16 or the camera 16 canbe moved relative to the fixed objective 31 along the optical axis. Forthe preferably electronically controlled length adjustment mechanism,all known drive technologies are suitable, such as spindle drives,stepping motors, direct-current motors, piezo actuators, etc. Thevariation of the distance between autofocus objective 31 and camera 16is only one example of the general principle of length adjustment ofcamera 16 relative to the image plane of the autofocus imaging. In thesimplest case, the longitudinal shifting mechanism can effect a shift ofthe camera 16, the imaging optical unit 29 or the light modulator 12.

This development removes the described limitation of the capture rangeor holding area.

The distance between autofocus objective and autofocus camera is nowadapted to the desired holding area such that the focus of the contrastsignal always lies in the center of the image field. For this, the stepsdescribed in WO 2007/144197 A are carried out.

In principle, the structured autofocus illumination, e.g., by means ofLCD or DMD, can also be realized with a transmitted light microscopeand/or as dark field illumination. However, in the case of transmittedlight, use is then limited to transparent sample holders as well as toscattering or fluorescent samples.

In all variants of the autofocus and tracking system, a computationalcontrol and evaluation device (e.g., a computer) is used which carriesout the signal analysis and the control of the actuator(s) (z-drive,xy-table, filter, etc.). Evaluation and control can be implemented byfirmware and/or software technology. This control/evaluation devicecarries out all sequential control described here.

Instead of adjustable, e.g., electrically switchable, light modulators(e.g., LCD, DMD), static light modulators (transmission or phasegratings) can also be used. The projected modulation object can beshiftable on the sample side using tiltable plane-parallel plates orother devices. An exchange of the gratings can likewise be possible tovary the grating constants or structure. As explained, a two-dimensionalgrating structure can also be used which has several different gratingperiods, for example 2 to 10 striped gratings arranged next to eachother with different grating frequencies. Depending on the design, eachstructure can be provided with a comparison object of its own or alsoseveral or all of the grating structures can be provided with a commoncomparison object.

As the autofocus method can preferably (but not necessarily) functionwith two-dimensional (field) cameras, the most suitable grating for eachapplication case can then be chosen in the case of several grating linesby reading the corresponding camera lines, without the need to carry outmechanical changes (e.g., exchange of the grating).

The described principle of the light modulator which provides amodulation structure and a comparison structure can be used not only ina microscope such as described by WO 2007/144197 A1, but a microscopewith the design according to DE 10319182 A1 also comes intoconsideration. The disclosure of this published document, DE 10319182A1, is therefore incorporated by reference in full in this respect. Themodulation structure and the structure necessary for the comparisonstructure are then located on an edge of a confocal slit diaphragmrunning at an angle. The imaging of the modulation object as well as ofthe comparison object thus takes place mostly through the beam pathwhich is also provided for the sample imaging in the microscope.

Where method steps or particular methods or modes of operation aredescribed in this description, these are realized by a control devicebelonging to the autofocus device, e.g., like in the control device 26.Naturally, a control device present in any case in the microscope 1 canalso undertake the corresponding control tasks.

1. A microscope with an objective, which has a focal plane lying in asample space, and an autofocus device having: a light modulator which isdesigned for generating a luminous modulation object that isintensity-modulated periodically along one direction, an autofocusillumination optical unit which images the modulation object such thatan image of the modulation object forms in the sample space, anautofocus camera, an autofocus imaging optical unit which images theimage of the modulation object formed in the sample space onto theautofocus camera, and a control device which receives signals of theautofocus camera and is designed to determine an intensity distributionof the image of the modulation object and to generate a focus controlsignal therefrom, wherein the light modulator additionally generates aluminous comparison object which extends along the direction of themodulation object, the autofocus illumination optical unit also imagesthe comparison object such that an image of the comparison object formsin the sample space, the autofocus imaging optical unit also images theimage of the comparison object formed in the sample space onto theautofocus camera, and the control device is designed to determine fromthe signals of the autofocus camera an intensity distribution of theimage of the comparison object along the direction and thereby tocorrect the intensity distribution of the image of the modulation objectwith regard to reflectivity variations in the sample space.
 2. Themicroscope according to claim 1, wherein the comparison object comprisesa line or a rectangular field.
 3. The microscope according to claim 2,wherein the comparison object comprises a grating slit, a gratingconstant of which lies below a resolution limit of the autofocus imagingoptical unit.
 4. The microscope according to claim 3, wherein themodulation object comprises a grating slit, wherein a duty cycle andslit width of the grating slit of the modulation object and of thecomparison object are substantially the same.
 5. The microscopeaccording to claim 1, wherein the comparison object is formed by acombination of modulation object with a structure that is complementaryto the modulation object.
 6. The microscope according to claim 1,wherein the control device is adapted to image the images of comparisonobject and modulation object sequentially and to determine the intensitydistributions sequentially.
 7. An autofocus method for a microscope withan objective which images a sample, wherein the method comprises thesteps of: imaging onto the sample a luminous modulation object that isintensity-modulated periodically along one direction and forming animage of the modulation object in the sample, determining an intensitydistribution of the image of the modulation object and generating afocus control signal therefrom, imaging onto the sample a luminouscomparison object which extends along the direction of the modulationobject and forming an image of the comparison object in the sample, anddetermining an intensity distribution of the image of the comparisonobject along the direction and correcting the intensity distribution ofthe image of the modulation object on a basis of the intensitydistribution of the comparison object with regard to reflectivityvariations of the sample.
 8. The autofocus method according to claim 7,wherein the comparison object comprises a line or a rectangular field.9. The autofocus method according to claim 8, wherein the comparisonobject comprises a grating slit a grating constant of which lies below aresolution limit of the autofocus imaging optical unit.
 10. Theautofocus method according to claim 9, wherein the modulation objectcomprises a grating gap, wherein a duty cycle and grating gap width ofthe grating gap of the modulation object and of the comparison objectare substantially the same.
 11. The autofocus method according to claim7, wherein the comparison object is formed by a combination of themodulation object with a structure that is complementary to themodulation object.
 12. The autofocus method according to claim 7,wherein the images of the comparison object and modulation object areimaged sequentially and the intensity distributions are determinedsequentially.